Optical distance measuring equipment is well known and has enjoyed widespread use, particularly in the field of digital image processing. This equipment typically employs a light source for illuminating the target points of interest. The light source is modulated by a reference signal at a given frequency. Detector means are provided for receiving light reflected from the target and generating an information waveform in response thereto. The distance between the light source and the target is obtained by measuring the phase shift between the reference and information waveforms.
The Environmental Research Institute of Michigan, the assignee of the present invention, has developed an improved image sensor which employs the above-described technique to generate a multi-bit digital pixel value representative of various characteristics, such as the distance between the image sensor and the target. The image sensor includes phase detector circuitry employed to detect the phase difference between the reference and information waveforms. This phase detector employs comparator circuits for converting the reference and information waveforms into digital logic level signals and a multi-bit digital counter, driven at a selected clock frequency by a phase locked loop frequency multiplier. The counter is enabled to begin counting at the clock frequency upon receipt of an edge of one of the digital levels and is reset upon the occurrence of a corresponding edge of the other digital logic level signals. Storage means is also provided for storing the count on the counter output after the counter has stopped. This storage means thus provides a multi-bit digital pixel value representative of the phase difference between the signals.
Various means have been utilized in the past to increase the level of precision of phase measurement devices. U.S. Pat. No. 3,728,025, issued to Madigan et al, discloses optical distance measuring equipment which transmits and receives three signals of varying frequency. The signals are so spaced in frequency range that a digital processing circuit provides coarse, intermediate and fine distance measurements.
U.S. Pat. No. 4,297,030, issued to Chaborski, discloses that the precision of a phase detector employing a digital counter may be increased by frequency multiplying the signal that drives the counter.
Unfortunately, the level of precision for each of these devices is limited by the maximum counting rate of the high speed digital counters.
It is an object of the present invention to provide means for extending the precision of phase measurement devices.